LIGO diary--report #3

by Wm. Robert Johnston
last updated 1 August 2001

Friday, 13 July 2001: First, a clarification of a statement the last report: after the laser beam is split into two beams sent down the two arms, each beam is trapped not between the beam splitter mirror and end station mirror, but between an additional mirror just after the beam splitter mirror and the end station mirror. (I know all of you recognized my slip, having drawn on your knowledge of Michelson-Morley interferometers and Fabry-Perot cavities, but I thought I would mention it anyway.)

Yesterday I tagged along on a formal tour of the LIGO facility. This tour was for a group of students from Louisiana State University in Baton Rouge. LSU is engaged in some related work: at least one researcher there is doing research for the next generation of LIGO mirrors. LIGO is slated to be fully upgraded in about 5 years, and part of the current plan is to use 40-kilogram sapphire mirrors. LSU also has a resonant bar detector for sensing gravitational waves (GWs) (I referred to these in the last report).

Anyway, the tour took us through the LVEA (which is Laser/Vacuum Equipment Area) room (most of which I had seen) plus up to the roof (where I had not been). From the roof you can look along the two arms, one going 4 kilometers west-southwest and one going 4 kilometers south-southeast. Each arm contains the vacuum tube, 1.22-meter diameter (outside) stainless steel tubes. The tubes are covered by concrete (to prevent, among other things, bullet holes from hunting rifles). Parallel to each arm is a water-filled ditch, left by the removal of soil to build up the height of the arm foundations.

The surrounding forest is owned mostly by timber companies, who periodically harvest sections of the woods. Thus, the trees come in patches of varying height (representing varying stages of regrowth following harvesting). I have seen grazing cattle in a least one section along the road approaching LIGO. I have also noticed the presence of small struggling palm trees at the edges of this pine forest.

We were also escorted into the room containing the laser itself. In there, we were able to use a camera operating similar to night vision goggles, through which we could see the otherwise invisible laser light in several locations.

The total volume of LIGO's vacuum assembly is about 8,000 cubic meters, equivalent to a cube 20 meters on a side. This is not the largest vacuum chamber in the world (NASA's Space Power Facility in Ohio has a volume about three times greater), but the two LIGO facilities are probably the largest ultrahigh vacuum facilities in the world. The quality of the vacuum is a few hundred times better than space at the altitude of the International Space Station.

Yesterday and today I assisted in a few more measurements of radio frequency emissions in the LVEA. Currently the plan is to try next week to isolate the source of the emissions. Between a presentation Wednesday and one today, I am a little more clear on one topic:

The laser beam, before reaching the beam splitter mirror, is modulated by an electric field. The effect of this is to produce two radio wave beams travelling along with the laser beam (which, recall, is infrared). These beams are reflected from the first mirror along each arm, a short distance from the beam splitter mirror. Only laser light (no radio waves) are trapped in each arm in the 4-kilometer space between mirrors. The radio wave signals must be electronically processed to produce a signal which is then used to constantly make microscopic adjustments to the positions of the mirrors.

Wednesday night I went to dinner with seven other students working here at LIGO. They are a diverse group, some from Louisiana and some from across the country, some graduate students but a majority undergraduate, and I believe all are majoring in physics.

We have had scattered thunderstorms again the last few days, including a severe thunderstorm Wednesday while driving back to Hammond.